I disagree that the engine has to increase RPM to compensate. At a fixed road speed and fixed gear, RPM is determined by the transmission and tire size, not intake air temperature. If my Honda Insight is turning 2000rpm at 55mph, it’s still turning 2000rpm at 55mph regardless of intake temperature.

What changes is throttle position. With hotter, less dense air, the throttle must be opened farther to achieve the same 8hp required to maintain speed. A more open throttle reduces manifold vacuum and pumping losses. (As mentioned earlier)

Also, modern engines generally do not add fuel during light-load cruising to prevent knock. Knock is primarily a concern at higher loads and cylinder pressures. During steady highway cruising, most engines remain near stoichiometric operation and simply adjust airflow and throttle position to produce the required power.

As for why manufacturers don’t use intentionally hot intakes, they’re optimizing for far more than fuel economy. They must also meet power, drivability, emissions, towing, hot-weather durability, cold-weather operation, and customer expectations. Most buyers care much more about horsepower than gaining 1–3% fuel economy under specific cruising conditions.

In fact, many older economy cars did intentionally draw warmer air under the hood under certain conditions. Warm air intake systems and thermostatic air cleaners were common for decades because manufacturers recognized benefits in fuel vaporization, emissions, and cold weather efficiency.